[Tommy]

You're right in thinking that there is only one pilot-in-command and he decides who is flying the airplane. But a good co-pilot can and should call out problems like low airspeed and insufficient traffic spacing. I'll add that if I were a co-pilot, I would physically intervene if it appeared the PIC's actions were about to kill me. After all, I'd rather be a live co-pilot without a job than a dead man.